Energy loss of highly charged ions in graphene

Abstract

The stopping forces exerted on ions as they pass through matter are a vital aspect of many appli- cations that involve ion-solid interactions [1, 2]. While ion stopping has been extensively studied for bulk solids, data for target materials that ap- proach the 2D limit remains scarce, especially for highly charged ions. When a highly charged ion impacts a sufficiently thin material it under- goes energy loss and charge exchange processes that result in a spectrum of outgoing ion charge states [3]. Here, we present experimental mea- surements and theoretical calculations of the en- ergy distribution of highly charged xenon ions transmitted through graphene. An electron beam ion source is used to pro- duce ionised xenon with charges of up to q = 28, which are then simultaneously energy and charge-state filtered using a Wien filter and trans- mitted through free-standing graphene. Energy distributions are measured using an in-house de- signed electrostatic analyzer placed behind the sample, which is able to resolve independent en- ergy distributions for each outgoing charge state. Additionally, transmission of xenon ions through graphene with incident charges of up to q = 12 is modeled using Time-Dependent Density Func- tional Theory (TDDFT) in the Ehrenfest dynam- ics approach [4]. We find that the ion energy loss in graphene is dependent on ion velocity, and that energy loss is greater for lower outgoing charge states for a given incident ion charge (shown in Figure 1). For xenon with q = 28, we measure a slight increase in kinetic energy after transmission for outgoing charge states larger than 14, although this is likely influenced by micro-scale defects in the graphene sample. The calculations pre- dict significantly higher energy loss compared to Figure 1: Experimental (solid lines) and simu- lated (points) energy loss as a function of outgo- ing charge state for Xe12+ in graphene. the experimental values and exhibit a limited ca- pacity for ion neutralisation, i.e. there are no predicted outgoing charge states below a thresh- old, and this threshold is not affected by reduc- ing ion velocity. Interestingly, calculations us- ing neutral xenon incident on graphene result in energy loss predictions closer to the experimen- tal ranges. The challenges in reconciling exper- iment and simulation highlights the complexity of charge-exchange processes of highly charged ions impacting surfaces, and suggests that the current state-of-the-art theory is yet to fully cap- ture the essential physics of highly charge ion and 2D material interactions. References [1] N. Hamada et al. J. Radiat. Res. 51 4 (2010) [2] X. Qi and F. Bruneval Phys. Rev. Lett. 128 043401 (2022) [3] R. A. Wilhelm, et al. Phys. Rev. Lett. 119 103401 (2017) [4] V. Zobacˇ, et al. J. Chem. Phys. 162 124117 (2025)